The use of a mode-locked ultrafast laser to generate a microwave frequency comb (MFC) in the tunneling junction of a scanning tunneling microscope has been described by this Inventor [M. J. Hagmann, A. J. Taylor and D. A. Yarotski, Observation of 200th harmonic with fractional linewidth of 10−10 in a microwave frequency comb generated in a tunneling junction,” Applied Physics Letters 101 (2012) 241102]. The inventor has also described how measurements of the MFC, as it propagates into a semiconductor as the sample electrode, may be used to determine the carrier density in a small region of the semiconductor that is close to the tunneling junction [M. J. Hagmann, P. Andrei, S. Pandey and A. Nahata, “Possible applications of scanning frequency comb microscopy for carrier profiling in semiconductors,” journal of Vacuum Science and Technology B 33 (2015) 02B109; and, U.S. application Ser. No. 15/448,151, filed 2 Mar. 2017, both of which are incorporated by reference herein in their entirety]. These previous disclosures all had two fundamental limitations.
The first limitation is that in previous studies of the MFC using a semiconductor as the sample electrode it was necessary to use lasers having a photon energy lower than the bandgap energy of the semiconductor. Otherwise electron-hole pairs form and separate to cause surge currents at the same frequencies as the harmonics of the MFC. For example, a Ti:Sapphire laser was used with metal samples or gallium nitride but an infrared laser is required with a silicon sample. [M. J. Hagmann, S. Pandey, A. Nahata, A. J. Taylor and D. A. Yarotski, “Microwave frequency comb attributed to the formation of dipoles at the surface of a semiconductor by a mode-locked ultrafast laser,” Applied Physics Letters 101 (2012) 231102]. The surge currents are independent of the tunneling process and have harmonics with greater power than those of the MFC because they are generated over the much larger surface of the semiconductor sample when it is exposed to the laser radiation.
The second limitation is that in previous studies of the MFC it was necessary to use lasers that are passively mode-locked, such as with a saturable absorber, Kerr-lens mode-locking, or with other nonlinear optical effects, so that no electrical signal is present at the laser pulse repetition frequency. By contrast, in active mode-locking an electrical signal at the pulse-repetition frequency is applied to an acousto-optic modulator or other device to modulate the laser. The different harmonics of the MFC typically have a power from −120 to −152 dBm at harmonics with progressively higher frequencies. However, the harmonics of the MFC may be measured with high accuracy because the signal-to-noise ratio is approximately 25 dB due to their extremely narrow (sub-Hz) linewidth. Nevertheless, with active mode-locking the leakage of the modulating electrical signal prevents measurements of the harmonics of the MFC.
In addition, the use of a modulating potential increases the signal-to-noise ratio and provide greater sensitivity greater precision in measuring the MFC. This allows operation of the STM at lower tunneling currents and cause less perturbation to the structure of the semi-conductor. This should be understood by those skilled in the art of phase-sensitive detection
The present invention is an improvement to the apparatus generating and measuring the MFC to allow for the use more general mode-locked lasers (active or passive) having an arbitrary carrier frequency.